The method presently used for etching through substrate vias in Gallium Arsenide (GaAs) is to use a chlorine-based plasma that kinetically and chemically attacks the GaAs. A bias voltage directs the plasma ions with some kinetic energy into the hole as it etches at a given etch rate, giving a high aspect ratio (steep sidewall) hole. The chemical reaction helps remove GaAs. The bias voltage, which is across the plasma and not the device can be greater than 100 V at acceptable etch rates. In the case of Indium Arsenide (InAs), this voltage associated with the plasma is enough to exceed the breakdown voltage of the high electron mobility transistor (HEMT), thereby damaging or destroying the device. The standard via etch cannot be used on InAs-channel HEMT MMICs. Furthermore, as etch rates increase the bias voltage can increase resulting in damage to other types of HEMT devices such as Indium Gallium Arsenide (InGaAs) and GaAs based devices.
InAs-channel HEMTs have superior low noise, low power consumption, and high speed due to the “supreme electronic properties of InAs” such as electron mobility and peak velocity. Table I shows 2.5× mobility and 1.5× velocity compared to InGaAs-channel devices, and even greater performance relative to GaAs. The high peak electron velocity at a low electric field enables the operation of InAs HEMTs at very low drain voltage, and therefore at very low power consumption.
TABLE IHEMT Channel Electronic Material PropertiesPropertyInAsIn0.53Ga0.47AsGaAsEffective Mass0.0230.0410.067Mobility20,0008,0004,500Peak Velocity4.02.72.2Valley0.900.550.31Separation (eV)Band Gap (eV)0.360.721.42
The high speed, low power consumption advantages are, however, offset by a size disadvantage. The InAs-channel HEMTs presently have to be made in a coplanar transmission line environment, with no through substrate vias. This is because of the low band gap of InAs (last row in Table I). The standard plasma etch via process results in a voltage on the HEMT contact pads that damages the low band gap device. The solution thus far has been to abandon the use of vias and use coplanar transmission lines which are noted for their large ground plane areas on the top surface of the substrate.